Esters derived from carboxylic acids or anhydrides and aliphatic alcohols comprise a large and broadly used class of compounds. Esters of phthalic anhydride, for example, are produced in quantities or more than 1.5 billion pounds per year. Of this group, di-2-ethylhexyl phthalate (DEHP) is the most commonly used member, and is produced in excess of 500 million pounds per year. The largest single use of DEHP, and of several other members of the group, is as a plasticizer for polyvinyl chloride resins. They are also used broadly as solvents, lubricants, functional fluids, additives and the like. A variety of other acids, for example, adipic acid, azelaic acid, terephthalic acid, trimellitic anhydride, benzoic acid and aliphatic acids of both natural and synthetic derivation, are extensively used in ester production. These acids are reacted with aliphatic alcohols containing 4 to 13 carbon atoms in the alkyl chain and 1 to 4 hydroxyl groups, to produce esters having an extremely broad range of properties and applications.
Large scale synthesis of esters began early in the 20th century and grew rapidly, along with related industries, to satisfy needs of plasticization, lubrication, hydraulics and the like. Along with this growth came a need for improved methods of manufacture. In most instances practical synthesis of esters requires catalyzed reactions of the carboxylic acid or anhydride with an excess of alcohols at elevated temperatures. Earlier commercial synthesis procedures usually used strong acids, as for example, sulfuric acid, p-toluene sulfonic acid, or phosphoric acid as catalysts. This type of reaction is carried out at temperatures of 130 to 160 degrees C. Conversions of carboxylic acid to ester are generally 95 to 98 percent complete and reaction mixtures are refined by washing with aqueous caustic to remove residual acids and to help improve color. Excess alcohol is removed by vacuum and steam distillations. Conversion of raw materials to ester is reduced both by acid catalyzed decompositions reactions and, to a greater extent, by losses in washing and filtering procedures.
A significant improvement in commercial esterification procedures was realized with the introduction and use of organometallic compounds as catalysts. These generally require higher temperatures than acid catalysts, typically from 190.degree. to 230.degree. C., but can provide a higher yield of esters due to less tendency for decomposition reactions as well as permitting higher conversions; giving 99.9 percent or more of conversion of acid to ester, compared with 98 to 99 percent by previous acid catalyzed methods. The most successful and widely used of the organometallic catalysts are tetra-alkyl titanate esters as described in Werber in U.S. Pat. No. 3,056,818. While other improvements in ester manufacture have been made, particularly in raw material quality and reactor design, washing procedures still are required in commercial esterification processes in order to provide high purity, commercial grades of DEHP and other esters. Such washing is required in order that the titanium catalysts residues may be sufficiently hydrolyzed to avoid (1) increase of acidity during steam distillation, (2) decrease of filtration rate due to coating of filter surfaces with gelatinous catalyst residues, and (3) cloudiness appearing in products after filtration due to imcomplete removal of titanium residues.
This invention provides a new and novel means of removing these titanium catalyst residues and providing high purity esters without the necessity of aqueous washing methods.